The Devil's Quirky Mind
Before boring you with an extremely long talk about Hiruma's personality and general behaviour, I want to say one thing straight out: I firmly believe that there isn't one single person in this world that can completely understand Hiruma's mind with the only exception of Inagaki and Hiruma himself.
If one could draw his thoughts, I'm sure there wouldn't still be one single maze complex enough to rappresant them.
This said, I'll just add that most of what I'm going to say next is bound to be my personal interpretation of his character (based on his actions, but still my personal interpretation).
The Devil in people's eyesWhen reading the manga the first time, there are two things about Hiruma's character that immediatly strikes the reader: his flashy reactions and his apparently erratic modus operandi. The whole world seems to be his playground as he coerces people to do whatever he wants by blackmailing them.
He's way too bossy and easily irritated by people stupidity, and he has in himself a good dose of "ahaha, it's so easy to trick you" kind of childishness.
Though prone to rage and excessive violence (to the point of using weapons against his own team-mates), he's also very canny, clever and intuitive, and he's a good mind that quickly gives him solutions about any problem he's facing.
Not-so-devilishBecause of his selfish behaviour and habit of exploiting everyone in general, most people think of him as a kind of "demon in human disguise" (in the first chapter of the series there are even people screaming in despair and tring to run away just because Sena and Mamori mention him aloud), easily forgetting that he's still a 17-years-old boy, though a very intelligent one.
Everyone takes for granted that he's unreachable and untouchable by feelings like sadness or that he can still have his own (twisted) sense of honour.
In my opinion he's just introverted almost to the point of paranoia and he just hates the idea of showing his feelings to others: when the few people that notice his potential uneasiness (mostly Musahi and Mamori, and later in the series also Sena) point it out, instead of acting with his usual irritated-and-noisy self, he just shrug it off, belittling whatever they're trying to say or just making a sarcastic remark out of it.
The Devil and his comradesMost of his "human feelings" are triggered by his two friends Musashi and Kurita. Expecially when it comes to the first one, Hiruma seems to lose his composure quite often.
Both when Musashi leaves and comes back to the team, he shows two expressions that he never had before (and actually never shows again during the series): a kind of sad submission in the first case (when he usually struggles to death to get what he wants) and a mix of hope and surprise in the latter. He shows to care for both his friends, though sometimes in an apparently mean way (eg: he's the tendency of using his weapons against Kurita whenever he's afraid or get nervous when he's overthinking things).
When it comes to honour...Hiruma's also shown to have a somewhat honourable behaviour when it comes to things related to American football: though he loves bluffing and often tries to mislead the opponents by saying lies or acting the opposite of what he's feeling, he always plays by the rules. Also, before the summer that precedes the Autumn Tournament, he releases the members of the Zokugaku Chameleons from their slavery (they'd to be his slaves because they lost a bet with him) to allow them to train in view of the tournament.
When it comes to winning...One of the most peculiar aspect of Hiruma's behaviour, it's his recurrently mentioning the idea that if there is no chance of winning a match than there is no pointin at all in playing it. As at first glance this behaviour can seem kind of cold and reductive, it shouldn't be completely taken as a negative statement as more than once Hiruma seems to imply that if you want to fight and reach the top, you need the will to do so. If, to begin with, you don't have the will to win then you'll probably end up not winning at all.
This last aspect is further stressed when in the last chapters Hiruma mentions the fact that he's willing to "struggle to death" to reach the top.
Influenced by common peopleAnother trait of Hiruma's character is his almost excessive objectivness: if something isn't going to go as predicted, then there is no point in it, if the chance of winning are none, then there is no point in fighting. It can seem a defeatist behaviour, but in reality it's just plain objectivness and a lot of craving for doing something more than hearing the mere "oh, don't worry, you did your best anyway".
During the series, however, Hiruma shows a kind of developping and he arrives to the point of completely thrusting his team-mates a couple of times against his better judgment and objective vision (he even goes for an on-side kick tactics without telling them about it, and thrusting they'd understand what his intentions were).